FIG. 1 shows a prior art OFDM receiver 10. A baseband signal 12 enters a synchronization function 14, which serves to identify phase and frequency offsets in the incoming signal 12, where they are fed back to an NCO (not shown) or a phase rotator (not shown) which removes the offsets and frequency drifts from the synchronized signal. The phase and frequency corrected signal 15 is delivered to an FFT 16 which recovers the combinations of OFDM subcarriers which comprise the transmitted data. FFT outputs 17 are shown as signal 17a, comprising linear combinations of FFT output data having real and imaginary components. The FFT output 17 is provided to a channel estimation and equalization function 18, which produces output 19 compensated for channel phase and magnitude variations. Plot 19a shows the output 19 in a frequency vs real and imaginary amplitude view, and plot 19b shows the corresponding constellation diagram for 16-QAM, where each position in a 16 QAM constellation diagram represents 4 bits of data after decoding. The output 19 of the channel compensator 18 is fed to the soft constellation de-mapper 24, which performs the function of converting the constellation into corresponding data values, and this output 23 is fed to the de-interleaver and soft decoder 20, which performs data decoding resulting in output data 22.
FIG. 2 shows a preamble stream 25 for an OFDM packet. The packet 25 comprises a sequence of preamble tones P0 through P15 which form a first preamble 26 followed by a second identical preamble 28, which is followed by a third preamble 30, and finally the packet data 32. During the preamble times corresponding to preambles 26, 28, and 30 of packet 25, the synchronization function 14 and channel estimation function 18 of FIG. 1 make estimations of channel frequency offset, phase offset, and channel frequency transfer function, respectively.
FIG. 3 shows one implementation of a prior art packet detection and coarse frequency offset synchronizer such as 14 of FIG. 1. The synchronizer comprises two parts, a coarse frequency offset part 40, and a packet detection part 60. The frequency offset estimator 40 accepts as an input a stream of complex OFDM symbols 92 and a delayed version 42 of the same stream, where the delay is equal to the interval of a single preamble interval 26. The conjugator 52 has the function of inverting the imaginary part of the incoming stream such that a+jb becomes a−jb. The product of (a+jb)(a−jb) produces the signal power level a2+b2, since the same-position preamble symbols are identical other than the frequency offset generated phase shift component from the earlier symbol to the later symbol. Consequently, the multiplier 44 output contains an imaginary component corresponding to the amount of phase shift from a first preamble symbol to a second preamble symbol. The Phase Finder 46, which is implemented as a CORDIC generates an output 47 which represents the phase φ of the incoming multiplier 44 product. The frequency may be then be estimated from change of phase per sample Δφ/Δt. The output of CORDIC 46 is averaged 48 to generate a coarse frequency offset 50. This value is measured during the preamble interval and fed back to a numerically controlled oscillator (NCO, not shown) or phase rotator (not shown) to remove any frequency offset during the balance of the packet receive time prior to performing the FFT, where such frequency offset would result in an offset in the FFT 16 of FIG. 1 outputs.
The symbol timing may be extracted from the processing shown as packet detection system 60 of FIG. 3. The incoming stream of baseband OFDM symbols are delayed 62 by a time equal to a preamble interval, and the preamble stream 92 is multiplied 66 by a delayed preamble 63 and conjugated 64 to produce multiplier 66 output 67. This output 67 is averaged over an interval equal to the number of symbols in a preamble (shown as 16 symbols) to generate a value Cn 74, which represents the power level of the signal, as before. During the preamble interval, the multiplication of a current preamble symbol with the same symbol from a previous preamble results in the output 67 of the multiplier 66 representing the correlated signal power. The averager 70 sums the previous preamble values (shown for a 16 symbol preamble) to generate a power value Cn 74 whose value represents the noise plus interference component of the SINR value to be determined. The output 63 of the delay element 62 is multiplied by a conjugate 64 value 65 to produce a product 69, which is averaged over the same preamble interval by averager 72 to generate a signal plus noise power level 76. Since there is very little signal correlation from one symbol of a preamble to the next, the output Pn 76 provides an indication of the uncorrelated noise plus interference level, which includes thermal noise and noise due to interfering sources which is not correlated with one symbol shift, in contrast to the correlated value Cn 74 which indicates the correlated power level of the incoming stream during the preamble interval. Cn 74 and Pn 76 are ordinarily used to establish the symbol timing referenced to the preamble, and one such method is to divide 78 the absolute value of Cn 84 by the noise plus signal level Pn 76 to generate a figure of merit μ 85, and to associate packet detection 90 with μ 85 crossing some predetermined threshold using a comparator 88.
FIG. 4 shows the signals for the prior art packet detection system of FIG. 3. The packet preamble is shown as 120, while signal power 67 is shown as 122 and noise and interference power signal 69 is shown as 124. Output Cn 74 is shown as signal 126, and output Pn 76 is shown as signal 128, which both rise during second preamble time t2, which corresponds to interval 28 of FIG. 2. The ratio of Cn/Pn is shown on waveform 127, and when waveform 127 crosses threshold 125, start of packet 121 is indicated, while end of preamble/start of data/symbol timing may be detected by falling correlated signal waveform 122 edge 123.
The use of existing signals Cn and Pn is known in the prior art for symbol timing and packet detection, and it is also known in the prior art to change demodulation method and transmission speed based on error rate at the detector. It is desired to generate a SINR estimate using these signals for use in demodulation, particularly following the soft constellation demapping step, whereby the quantization method performed on the demapped data may be changed in accordance with the value of SINR as determined during the preamble synchronization step.
An estimate of the receiver signal quality can be used to improve the performance or reduce the complexity of base-band processing functions. An estimate of the noise variance is a sufficient measure of the signal quality, as the AGC (Automatic Gain Control) function of the RF receiver (not shown) ensures constant input power to a base-band system. Typically, symbol decisions are compared with the received symbol to obtain an error vector. The error vectors can be averaged to obtain an estimate of the noise variance as discussed in U.S. Pat. No. 5,379,324. The symbol decisions can be made at the input to the decoder, or at the decoder output. Using decisions from the output of the decoder provides a better estimate of the noise variance. Both these techniques have significant latency, and it is useful to have an estimate of signal strength established during the preamble interval so that it may be used during the data interval of the same packet. It is desired to have a signal strength estimation for use in an OFDM system which relies on parameters which can be established during the preamble interval.
A technique for synchronization based on a training sequence consisting of repeating patterns is described in “Robust Frequency and Timing Synchronization for OFDM”, IEEE Transactions on communications, December 1997. As noted in FIG. 3 and FIG. 4, due to the repeating preamble symbols, a correlation peak is observed at the end of the training sequence. This peak is used to detect a valid reception. The position of the peak also indicates the symbol boundary.
The correlation be represented as,
      C    ⁡          (      n      )        =            ∑                        n          -          L                <        k        ⁢                  <          _                ⁢        n              ⁢                  ⁢                  X        ⁡                  (          k          )                    *                        X          ⁡                      (                          k              -              L                        )                          *            The signal energy is computed as,
      E    ⁡          (      n      )        =            1      2        ⁢                  ∑                              n            -                          2              ⁢              L                                <          k          ⁢                      <            _                    ⁢          n                    ⁢                          ⁢                                              X            ⁡                          (              k              )                                                2            The normalized value used for symbol timing is given by
      Y    ⁡          (      n      )        =                                      C          ⁡                      (            n            )                                      2                      E        ⁡                  (          n          )                    2      
FIG. 6 shows a prior art wireless LAN (WLAN) 602 which includes an access point (AP) 614 and a plurality of stations STA-1 604 through STA-5 612. Each station such as 604 has a related wireless communications link such as 616, and the speed of each link is determined by a negotiation process that includes the station 604 capabilities and quality of link 616. The quality of the link 616 may be dependant on the amount of multi-path interference, or link distance and attenuation, or any number of factors. In the prior art, the link speed 616 is reduced when the link quality is degraded. Each station 606, 608, 610, 612 that is part of the access point 614 may operate at an independent data rate on link 618, 629, 622, 624 within the wireless region 602, and for 802.11a or 802.11g which use Orthogonal Frequency Division Modulation (OFDM) for modulation and demodulation, the available data rates are 6 Mbps, 9 Mbps, 12 Mbps, 18 Mbps, 24 Mbps, 36 Mbps, 48 Mbps and 54 Mbps.
A problem arises when the SINR falls below an acceptable threshold for an outlying station such as Station 3 608, shown operating at the minimum rate of 6 Mbps. Below a critical noise threshold, the error rate for incoming demodulated data to the station will be high. In the wireless LAN protocols, each transmitted packet is explicitly acknowledged by the receiver, and when the acknowledgements are not received by the transmitter, a presumption is made by the sender that the packet was not received, and the packet is retransmitted. Since the transmitter has no knowledge of the SINR at the receiver, and a single bit error results in the retransmission of the entire packet, a problem in low SINR environments is that power resources are consumed in battery powered stations such as STA-3 608 attempting to recover or retransmit data in SINR environments where it is unlikely that an entire message formed from a plurality of packets will be successfully received.
FIG. 7 shows a prior art wireless data stream 700, whereby packets 702, 704, 706, 708 are received for a particular station such as 608 of FIG. 6. An inter-packet gap 710 is present which may be short for back-to-back transmitted packets of a single message, or very long after completion of a current message and prior to the next message.
FIG. 8 shows a prior art wireless receiver for OFDM, which includes an antenna 822, an analog front end 800 performing low noise amplification and mixing from the modulation frequency to baseband, analog to digital converter 804, phase correction 806, and symbol timing detector 808, shown for simplicity as packet detection processing 818. After packet detection is accomplished, the data is Fourier transformed 810, phase equalized 812 to recover the subcarriers of the modulation, demodulated 814, and decoded 816, each step of which is well known to one skilled in the art of wireless OFDM systems.
U.S. Pat. No. 5,214,675 by Mueller et al. describes a system for compensating for multi-path reflection in a communications system by computing a variance of the signal and providing this signal to a filter which compensates for multipath delay.
U.S. Pat. No. 6,792,055 by Hart describes a system for use in QAM whereby the strength of the demodulated signal is fed back to a gain control. In another embodiment, the decoder makes hard and soft decisions according to a variable threshold which is set by the strength of the signal applied to the decoder.
U.S. Pat. No. 5,740,203 describes a prior art demapper for QAM and PSK modulation methods which performs the function of block 24 of FIG. 1 or block 140 of FIG. 6.
U.S. Pat. No. 5,379,324 by Mueller et al describes a system for computing gain and noise variance of a channel for use in correcting the channel.